Method for sensing rotation using vibrating piezoelectric elements

ABSTRACT

A vibratory angular rate sensor system preferably consists of a Z-cut quartz plate forming a mounting frame with a rectangular opening. Within the opening are mounted two pairs of tines. Each pair of tines is parallel to each other, one pair forming the drive tines and the other pair the output tines. Each corresponding set of two tines is disposed along the same axis having a common stem or base. The tines are secured by four bridges integral with the frame and connected to the stem. The arrangement is such that the pair of input tines vibrates in opposition to each other, while the pair of output tines vibrates with one tine going up while the other moves downwardly. As a result, the angular rate sensors drive frequency and the structural torque frequency are unequal. Therefore large displacements of the stem are unnecessary.

This is a continuation of application Ser. No. 733,719 filed May 13,1985, abandoned, which is a division of application Ser. No. 572,783,filed Jan. 23, 1984, now U.S. Pat. No. 4,524,619.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application may be considered to be an improvement and anextension of the principles of a prior application entitled, "AngularRate Sensor System," to Alsenz, at al., Ser. No. 06/321,964, filed Nov.16, 1981. The present application is assigned to the same assignee as isthe prior copending application.

The present application is also related to a copending application toJuptner, et al., entitled, "A Vibratory Angular Rate Sensing System,"filed concurrently with the present application. The present applicationdiscloses a different configuration which substantially cancels allundesirable first and second harmonics of the output frequencies whichrepresent noise.

BACKGROUND OF THE INVENTION

The angular rate of motion of a craft is an essential input for allnavigational and inertial guidance systems. Such systems are usedconventionally for aircraft, spacecraft, ships, or missiles. The sensingof the angular rate of motion is presently accomplished by means of agyroscope.

Gyroscopes, however, have various disadvantages. They must be built toextremely high accuracies and may have drift rates of fractions of adegree per hour. Due to the expense of building them, they are verycostly; they are physically large and heavy. They must be frequently andprecisely maintained, for the reason that critical movable elements,such as bearings, may change with time. They may also be damaged by evenlow levels of shock and vibration. This, in turn, may cause an increaseof unknown size in the drift rate, occurring at unknown times.

Because gyroscopes are sensitive to the effects of shock and vibration,they frequently have heavy mounting configurations to protect them,which also are expensive.

SUMMARY OF THE INVENTION

It will accordingly be obvious that it is desirable to replace agyroscope by some other device which is less expensive and which iscapable of measuring angular rates, thereby measuring the attitude of avehicle or craft. In accordance with the present invention, this isaccomplished by a balanced resonant sensor. Such a sensor isrepresented, in accordance with the present invention by a tuning fork.The tuning fork should be substantially mechanically temperature stable,having low internal friction and following Hook's Law. According toHook's Law, the strain set up within an elastic body is proportional tothe stress to which the body is subjected by the applied load (thestrain, however, must be within the elastic limit of the body), and thebody will return to its original shape when the stress is removed.

Preferably, but not necessarily, the tuning fork consists of quartz.However, other piezoelectric materials may be used, such as syntheticcrystals; for example, ethylene diamine tartrate (EDT), dipotassiumtartrate (DKT) or ammonium dihydrogen phosphate (ADP). Non-piezoelectricmaterials may by used with an electromagnetic drive.

In accordance with the present invention there is provided a wafer ofpiezoelectric materials, preferably of Z-cut quartz. The wafer is cut toprovide a frame having an opening within which are provided two pairs oftines. The two tines in each pair are parallel with each other and areinterconnected by a stem. This vibratory structure is secured to theframe by a pair of suspension bridges disposed close to each other,integral with the frame and extending to the stem.

The first pair of tines is excited by a drive oscillator in such amanner that the two tines will move toward each other and, after aninstant, away from each other. The other pair of tines represent theoutput tines, and they will vibrate due to an applied external force insuch a manner that while one tine moves up, the other moves down, andvice versa.

The novel features that are considered characteristic of this inventionare set forth with particularity in the appended claims. The inventionitself, however, both as to its organization and method of operation, aswell as additional objects and advantages thereof, will best beunderstood from the following description when read in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view of a vibratory angular rate sensor systemembodying the present invention;

FIG. 2 illustrates the triagonal crystalline orientation of a Z-cutquartz wafer;

FIG. 3 is a cross-sectional view taken along lines 3--3 of one of thefirst pair of tines and showing the drive electrodes as well as thedrive oscillator;

FIG. 4 is a schematic view of the pair of electrodes of FIG. 3 and adrive oscillator;

FIG. 5 is a similar cross-sectional view to that of FIG. 3 but takenalong lines 5--5 through one of the tines of the second pair of tinesand showing two pairs of output electrodes; and

FIG. 6 is a schematic view of the pair of electrodes of FIG. 5 and anoutput circuit.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring now to the drawing and particularly to FIG. 1, there isillustrated a vibratory angular rate sensor embodying the presentinvention. The sensor system includes a wafer 10 forming a mountingframe which preferably is a piece of Z-cut quartz. The crystallineorientation of the quartz is shown in FIG. 2. The mounting frame 10 isof rectangular configuration and has a rectangular central opening 11.The orientation of the wafer is shown adjacent to FIG. 1, illustratingthe X, Y, and Z axes. Disposed within the opening 11 is a first pair oftines 12 and 13, arranged substantially parallel to each other. Thesetines 12, 13 represent the input or drive tines. A second pair of tines14 and 15 are the output tines and are also disposed parallel to eachother and have common axes with the respective tines 12, 13. The twopairs of tines 12, 13 and 14, 15 are interconnected by a stem or base16.

The resonant system of the two pairs of tines 12, 13 and 14, 15 (theformer being the input tines and the latter pair forming the outputtines) is secured to the frame by four suspension bridges 17, 18. Thesuspension bridges 17, 18 are integral with the frame 10 and are securedto the stem of base 16.

By way of example, the side of the frame 10 may have a length of 0.400inch (in the X direction), the long side may have a length of 1.050 inch(in the Y direction) and the thickness (in the Z direction) may amountto 0.020 inch. However, it will be understood that other dimensions maybe used for different materials or other purposes.

The tines 12, 13 may be energized by a pair of electrodes illustrated inFIG. 3. Thus, there are two pairs of electrodes 20, 21 and 22, 23. Eachpair 20, 21 and 22, 23 is connected together and across a driveoscillator 25. (See FIG. 4). The electrodes 20, 21 extend in the Ydirection, while the electrodes 22, 23 extend in the Z direction.

As illustrated in FIGS. 5 and 6, the output signal is derived at boththe output tines 14, 15 by means of a first pair of output electrodes26, 27 and a second pair of electrodes 28, 30. The output signals fromboth output tines 14 and 15 are connected in parallel. For convenience,only the output electrodes and output leads of one of the output tineshave been illustrated. All of the output electrodes extend in the Y-Zdirection. The output electrodes 26 and 30 are connected together, whilethe other electrodes 27 and 28 are also connected together and across anoutput circuit 32. This may be any conventional output circuit as wellknown in the art.

The structure of FIG. 1 may be chemically etched by means ofphotolithography from a suitable quartz wafer, or else it may bemachined by a laser beam, ultrasonic machining, or other methods wellknown in the semiconductor art. The electrodes 20 to 23 and 26 to 30 maybe obtained by gold-plating the respective tines and by removingunnecessary portions of the gold film, for example by a laser beam or bychemical etching.

The tines 12 and 13 resonate in the fundamental flexural mode in the X-Yplane. This is shown by arrows 35 in FIG. 1 which show movement indirections lying in the plane of the paper. The frequency in the X-Yplane is substantially lower than that in the Y-Z plane; that is, theplane in which the tines 14 and 15 vibrate. Hence the tines 14, 15vibrate in opposite directions, up and down.

When the entire structure rotates about the Y axis and in the X-Y plane,the X-Y flexure plane vibration is conserved and therefore a flexure inthe Y-Z plane is initiated. This motion is represented by the arrows 36in FIG. 1 which show motion in directions perpendicular to the plane ofthe paper. The rotation in the X-Y plane may be caused by the angularmotion of the vehicle carrying the system. As a result, the stem 16 istwisted due to the Coriolis force which acts normal to the plane ofvibration of tines 12 and 13. This, in turn, causes an up-and-downmotion of the output tines 14, 15 in opposite directions.

It should be noted that the Y-Z flexural frequency is higher than theX-Y flexural frequency. Stated another way, a torque is felt by the stem16. This, in turn, initiates or drives an Y-Z flexure in output tines14, 15. This is so because the frequency is substantially similar tothat of the tuning fork consisting of tines 12, 13. The X-Y flexure intines 12 and 13 is piezoelectrically driven by the input electrodes 20to 23 of FIG. 3. On the other hand, the Y-Z flexure caused by a rotationabout the axis Y in tines 14, 15 is picked up piezoelectrically by theoutput electrodes 26 to 30 of FIG. 5.

The bridges 17, 18 have an X-Z flexural frequency which is substantiallythat of the flexural frequency of the tines 12, 13.

The electrodes and shielding connections for the input and outputcircuits are preferably made from the bridges 17, 18. It should be notedthat the frequency and balance of the two pairs of tines 12, 13 and 14,15 are adjusted by adding or removing material, such as a gold film, atthe free ends of the tines on the appropriate sides. This may beeffected by chemical etching or by a laser beam.

The rate sensors of the prior art depend on the flexural frequency ofthe drive tines being substantially the same as the torsional frequencyof the entire system. According to the present invention, the drivefrequency of the angular rate sensor and the structural torsionalfrequency are not the same. Therefore, large displacements of the stem16 are not necessary. The displacement of the stem 16 is extremely smallrelative to that of the pickup or output tines 14, 15. This is due tothe Ω multiplication of the displacement of the tines with respect tothe entire structure. It will now be understood that the vibration ofthe angular rate sensor is easily isolated from the mounting frame 10and hence from the environment.

What is claimed is:
 1. A method of sensing rotation, comprising thesteps of:vibrating a first pair of piezoelectric tines at a firstfrequency; and sensing piezoelectric voltages in a second pair of tinesparallel to and interconnected with said first pair of tines, saidvoltages induced in the second pair of tines by the vibration of saidsecond pair of tines being caused by the vibration of said first pair oftines and the rotation of said first and second pair of interconnectedtines.
 2. In a method for sensing angular rate by the use of a dual forkstructure having first and second interconnected forks lying in planesand carried by a support structure with the first fork having a majoraxis of symmetry, coupling energy into the first fork to cause vibratorymotion of the first fork in the plane of the first fork and sensingvibratory motion of the second fork in a direction normal to the planeof the second fork caused by vibration and rotation of the first fork toprovide a measure of the input angular rate about the major axis ofsymmetry of the first fork, said vibratory motions of the first andsecond forks being isolated from vibrations of the support structure. 3.A method as in claim 2 together with the step of operating the firstfork at a resonant frequency which is lower than the frequency of thesecond fork.